Test tube heater



Oct. 29, 1963 w. J. WALSH TEST TUBE HEATER 2 Sheets-Sheet 1 Filed Aug. 17, 1960 INVENTOR;

J. WALSH ILLJAM M%W EH7 5.

Oct. 29, 1963 w. J. WALSH 3,109,084

TEST TUBE HEATER Filed Aug. 17, 1960 2 Sheets-Sheet 2 I I I I 53 I //z :1 m I m 55 56 a 1 fi' *5: Q I '/zZ@ 7% :5 i Q I I W i 48 I IN V EN TOR.

BKIVILLIAM J. WALSH United StatesPatent O Illinois Filed Aug. 17, 1960, Ser. No. 50,182 5 Claims. (til. 2119-43) This invention relates to a new and improved test tube heater suitable for medical tests and other similar applications and to a new and improved electrical control device for test tube heaters and similar apparatus.

There are a number of applications in which precisely controlled heating of test tubes and other vessels is essential. In many instances, and particularly in medical test procedures, it is necessary to maintain the contents of the test tubes or other vessels at a constant temperature over an extended period of time. To conduct tests of this kind in an accurate manner, it is necessary to heat the contents of the test tube evenly. Previously known devices have, in at least some instances, presented substantial difiiculties with respect to uniform heating of the test tube contents; they tend to set up temperature gradients within the test tubes. It is also necessary, in many procedures, for the medical technician, chemist, or other person conducting the test to inspect the contents of the test tube at periodic intervals. This should be accomplished without removing the test tube from the heater or changing its temperature. This requirement conflicts with the requirement for uniform heating of the test tube contents, since exposure of any substantial portion of the test tube may cause localized cooling therein. The requirement for holding precise temperature control over long periods, which may extend for at least a number of hours, has also led to substantial difliculties in devices of this kind. The thermal-electrical control devices previously used in test tube heaters and in other similar applications are usually based upon the use of a temperature sensing element which is not physically a part of the heated device. Consequently, even though the temperature sensing element may be buried within the heated device, it does not always accurately reflect the actual temperature of the device. This problem is particularly acute with respect to apparatus, such as test tube heaters or specialized hot plates, in which an exact temperature must be held regardless of changes in ambient temperature or other external factors.

It is an object of the present invention, therefore, to provide a new and improved heater for test tubes and similar vessels which is effective to heat the contents of the test tubes evenly and completely and to hold the test tubes at a constant temperature over extended periods of time.

Another object of the invention is to permit convenient and effective inspection of the contents of the test tubes, while the tubes are being heated, without permitting localized cooling of the tubes.

Another object of the invention is to provide a new and improved test tube heater structure which can be easily and conveniently cleaned without requiring major disassembly of the heater.

Another object of the invention is to provide a new and improved thermal control for controlling the temperature of a test tube heater or similar device which is directly responsive to the temperature of the heated device and is not dependent upon an external sensing apparatus actuated by heat conducted thereto.

An additional object of the invention is to provide a high-sensitivity thermal electrical control adapted for either slow make-and-break operation or snap-action operation of the electrical contacts.

A heater for test tubes or similar vessels, constructed ice in accordance with the invention comprises a heater block which is preferably formed from a high-conductivity highexpansion metal such as aluminum. The heater block is provided with a plurality of elongated cavities, open at one end, for receiving individual test tubes. The depth of these cavities shouldbe such that a major portion of the length of the test tube is enclosed within the lock whenever the test tube is mounted therein. The heater block is further provided with a plurality of viewing apertures which extend through the block, each aperture being located at the base of one of the test tube receiving cavities. These viewing apertures permit visual inspection and cleaning of the base portion of each of the test tube cavities. Preferably, transparent covers are arovided on the heater block to cover the viewing apertures and prevent air circulation therethrough. The heater is also provided with a thermally controlled heating device, located at the base of the block, for heating the block and the test tubes to a predetermined temperature.

The preferred form of thermal control for the test tube heater, which may also be employed in other similar ap plications, comprises an actuating member having a thermal coefficient of expansion substantially different from that of the heater block or other workpiece. Preferably, two such actuating members are employed, and V-shaped actuating members may be utilized to substantial advantage. The actuating members are affixed directly to the heater block, at their opposite ends, the mounting of the actuating members on the block being such that the members are subject to substantial deformation due to mechanical stresses applied to the members as the direct result of expansion and contraction of the workpiece. The deformation of the actuating members is utilized to actuate a control switch which, in turn, controls a heater associated with the block.

Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawings which, by way of illustration, show preferred embodiments of the present invention and the principles thereof and what is now considered to be the best mode contemplated for applying these principles. Other embodiments of the invention embodying the same or equivalent principles may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention.

In the drawings:

FIG. 1 is a perspective view of a test tube heater constructed in accordance with one embodiment of the invention;

FIG. 2 is an elevation view, partly in cross-section, of the test tube heater in FIG. 1

FIG. 3 is a bottom view of the test tube heater with the support cradle and bottom cover omitted in order to illustrate the thermal control device of the heater;

FIG. 4 is an elevation view, partly in cross-section, of a test tube heater constructed in accordance with another embodiment of the invention;

FIG. 5 is a bottom view, with the bottom cover and support cradle removed, showing the control apparatus for the heater of FIG. 4; and

FIG. 6 is a bottom view, similar to FIGS. 3-5, illustrating another control apparatus constructed in accordance with the invention.

The test tube heater 10 illustrated in FIGS. 1-3 comprises a heater block 11 having a plurality of enclosed elongated cavities 12 for receiving individual test tubes. The heater block 11 is formed from a high-conductivity metal, such as aluminum. A preferred construction utilizes an aluminum block with a dark anodized external surface, preferably black in color. As best shown in and 12B at the ends of the heater block 11 in FIG. 2.

The base plate 13, which preferably is also fabricated from aluminum, comprises the top of a five-sided boxlike structure 16, the bottom of the base 16 being open. The base 16 affords a housing for the electrical thermal control of the test tube heater as explained in detail hereinafter.

in the test tube heater 16), the heater block 11 is provided with a plurality of viewing'apertures 17 which extend transversely through the block. The viewing apertures or windows 17 correspond in number to the test tube cavities 12 and afford a means for visual inspection of the base portion of each of the test tube cavities. A pair of transparent covers 18 and 19 are mounted on opposite sides of the heater block 11 by suitable means such as the mounting screws 21. The transparent covers 18 and 19 cover the windows 17 and prevent air circulation through the viewing apertures and through the test tube cavities 12 without interfering with visual inspection of the test tube contents. The transparent covers 18 and 19 should be fabricated from a heat-resistant material. For relatively low temperature applications, such as blood coagulation tests and other tests conducted below 100 C., a heat-resistant resin, such as the acrylic resins available under the trade names Lucite and Plexiglas, may be utilized for the covers. For high temperature applications, it may be necessary to adopt a high temperature glass or other material.

As noted hereinabove, the test tube heater 10 includes heating means mounted in the base 16 for heating the block 11 and any test tubes mounted therein to a predetermined constant temperature. In the test tube heater 10, the heating means comprises a resistance-wire heater 22 of relatively simple construction comprising an elongated length of resistance wire mounted upon an insulating base member. The resistance wire is disposed on the upper face of the heating member to afford good thermal conduction to the base plate 13 and, hence, to the heater block 11. It may be electrically insulated from the plate 13 by any suitable means, such as a layer of glass tape. Of course, the base plate 13 can be fabricated as an integral part of the heater block 11, if desired, but the illustrated construction affords adequate thermal conductivity and even heating of the heater block, and is less expensive and easier to clean than an integral construction.

The thermal control apparatus for the test tube heater 10 is best illustrated in FIG. 3 and comprises a pair of substantially V-shaped actuating members 24 and 25 which are mounted on the heater block 11 with the apex 26 of the member 24 located directly opposite the apex 27 of the member 25. As described in detail hereinafter, the control action in the thermal control apparatus of the heater 10 depends primarily on expansion of the block 11. To avoid substantial variations in control act-ion due to changes in ambient temperature, the "actuating members 24 and 25 are preferably fabricated from a ma terial having a low coeflicient of thermal expansion, the smaller the better. For example, the alloy known as N-ilvar is quite suitable; it has virtually zero coefiicient of thermal expansion. Of course, other means can be employed for ambient temperature compensation. Thus, if the base portion of the heater, containing the members 24 and 25, were insulated from the heater 22 and artificially held at a fixed temperature, the actuating members could be formed from virtually any material, but arrangements of this kind are more expensive and complex than the preferred construction shown herein.

The two actuating members 24 and 25 are substantially C-shaped in cross-section, to afford adequate rigidity, but are cut away at the central or apex portions thereof to permit the actuating members to bend and deflect toward and away from each other as explained hereinafter. The members 24 and 25 are not mounted upon the base plate 13, but are directly secured to the heater block 11. More specifically, one end of each of the actuating members is spot welded or otherwise securely aflixed to a mounting block 28, whereas the opposite end of each actuating member is similarly affixed to a second mounting block 29. The mounting lock 28 is affixed to the heater block 11 by means of a screw 32 that extends through the base plate .13 and is threaded directly into the heater block 11 (see FIG. 2). The same construction, using a mounting screw 33, is employed for the mounting block 29. The apertures in the base plate 13 through which the mounting screws 32 and 33 extend may be made slightly larger than the shank portions of the screws so that the base plate does not bind the screws. in effect, therefore, the mounting blocks 28 and 29 are made an integral part of the heater block.

A first cantilever spring contact arm 34 is mounted upon the actuating member 25 by suitable means such as a mounting bolt 36; an insulating bushing is utilized to insulate the contact arm 34 electrically from the actuating member. The spring contact arm 34 is electrically connected to one side of a power line 37. through the mounting bolt 36. A contact element 38 is preferably mounted at one end of the cantilever contact arm 34. An adjusting screw 39 may be mounted on the member 25 in position to engage the contact arm 34 and adjust its position relative to the actuating member.

A second cantilever spring contact arm 41 is mounted upon the actuating member 24; this contact arm is sub stantially similar to the spring contact arm 34 and is mounted upon the actuating member 24 by suitable means such as a mounting bolt 42. The contact arm 41 is electrically connected, through the bolt 42 to the resistance wire 43 of the heater 22. Suitable means are provided, of course, to insulate the contact arm 41 electrically from the member 24. A contact element 44 is provided at the end of the cantilever contact arm 41 and is normally disposed in engagement with the contact element 38 on the arm 34. A capacitor 45 may be electrically connected across the two contact arms 34 and 41 to reduce arcing between the cont-act elements 38 and 44 during operation of the control.

The base 16 of the test tube heater 10 could be left open but preferably is enclosed by a closure plate or cover 46 which is secured to the base by means of a plurality of screws 47 (see FIGS. 1 and 2). In the illustrated construction, the bottom plate 46 is provided with a pair of up-turned flanges which engage the bottom of the heater 22 and hold the heater in position against the base plate 13. A plurality of insulated feet 48 may be mounted upon the closure plate 46.

A pair of trunnion members 51 and 52 are mounted at the opposite ends of the heater block 11, as best shown in FIG. 2. The trunnion members '51 and 52 should be made of a heat-insulating material, and may be secured to the heater block by any suitable means such as the mounting screws 53 and 54. Each of the trunnion members 51 and 52 is provided with an annular recess in the central portion thereof, the recessed portion engaging aslot in one of the legs 55 and 56 of a support cradle 57. Thus, the heater block 11 is pivotally mounted upon the support cradle 57 to permit tipping of the heater block to any desired angle for inspection of the test tubes, such as the test tubes 14 and 15. This pivotal mounting also makes it convenient to agitate the heater block if required by the process in which the heater 10 is employed. The cradle 57 is provided with a plurality of insulated feet 58 to support the cradle upon any desired surface.

In the utilization of the test tube heater 1%, one or more test tubes are disposed in the heater block 11 in the manner illustrated by the test tubes 14 and 15 in FIG. 2. As noted hereinabove, the major portion of each test tube is encompassed within the heater block. Preferably, the level to which the test tubes are filled is kept below the upper surface 59 of the heater block. The power cord 37 is connected to a suitable power source, such as a conventional 115 volt AJC. supply. As a consequence, the heater 22 is energized, heating the block 11 and the contents of the test tubes. Because a major portion of each of the test tubes is fully encompassed within the heater block, heating of the test tubes and their contents is quite uniform. The condition of the test tube contents can be readily observed, at any time, through the transparent covers 18 and 19 and the windows 17 in the base of the test tube cavities. On the other hand, the covers 18 and 19 prevent any air circulation through the apertures 17, so that localized cooling of the test tubes, which might result from direct exposure to the air through windows of substantial size, is prevented. If the contents of any of the test tubes, or other foreign material, becomes lodged at the bottom of any of the test tube cavities 12, one or both of the transparent covers 13 and 19 can be conveniently removed from the heater block 11. With the transparent covers removed, the apertures 17 in the heater block afliord convenient access to the cavities so that they can be quickly and thoroughly cleaned. Moreover, all of this can be accomplished without disturbing the electrical control apparatus mounted in the base 1 6 in any way.

The contact elements 38 and 44 of the electrical control for the heater 1% (see FIG. 3) are normally engaged with each other. The energizing circuit for the heater 22 includes, in series, the contacts 33 and 44. As the heater 22 heats the block 11, the heater block expands. As the heater block expands, the two mounting blocks 28 and .29 are displaced away from each other, the displacement being directly representative of the expansion of the heater block because the two mounting blocks are directly secured to the heater block. -As the mounting blocks 28 and 29 move away from each other, due to thermal expansion of the heater block, the two actuating members 24 and 25 are deformed from their original configuration. That is, the elongation of the actuating members caused by the movement of the mounting blocks 28 and 29 tends to flatten the V-shaped actuating members and move their apex portions 26 and 27 away from each other. Accordingly, the two contact arms 34 and 41 move away from each other, and eventually, the contact elements 33 and 44 separate, opening the energizing circuit for the heater 22. As noted hereinabove, the capacitor 4-5 connected across the two contacts is efiective to minimize arcing between the contacts'when the circuit is opened.

When the contacts 38 and 44 have separated, and the heater 22 is de-energized, the block 11 starts to cool. As the heater block cools it contracts, and the two mounting blocks 28 and 2 9, which are in elfect a part of the heater block, move back toward each other. Consequently, the two V-shaped actuating members 24 and 25 return toward their original positions, relative to each other, with the result that the contacts 38 and 44 again engage. This restores the energizing circuit for the heater 22 and the heating cycle is repeated as described hereinabove.

The test tube heater 1!] can be utilized for heating test tubes to any fixed temperature within a relatively broad range by adjusting the initial position of the contact arm 34, using the adjusting screw 39. Thus, if the adjusting screw 39 is advanced, it tends to bend the spring arm 34 toward the arm 41. As the heater block 11 is heated, pulling the members 24 and 25 away from each other due to the mechanical stresses applied to the actuating members, the contacts 38 and 44 remain in contact until the free end of the arm 34 is straightened. Thereafter, continued deflection of the members 24 and 25 separates the two contacts. Further advancement of the adjusting screw increases the temperature at which the contacts separate. If the adjusting screw 39 is retreated, the disengagement of the contacts 38 and 44 takes place at a lower temperature.

It is important that the two actuating members 24 and '25 be directly and rigidly secured to the heater block 11. en this is done, the stresses applied to the two actuating members are directly representative of the expansion and contraction of the heater block 11 and, accordingly, are directly representative of the temperature of the heater block. Consequently, the operating temperature for the control system is not affected by changes in ambient temperature or by other external factors and is made much more sensitive to the actual thermal condition of the heater block or other workpiece than if the actuating members were controlled by a sensing device responsive to heat reflected from or otherwise conducted from the workpiece through some intermediate structure. Of course, it would be possible to utilize only one of the actuating members, such as the members 24 and 25, and to mount a second electrical contact directly on the base of the heater. The use of two V-shaped actuating members is preferred, .however, since this aifords a more sensitive control and makes it possible to hold a given temperature within closer tolerances.

As noted hereinabove, the pivotal mounting of the heater block 11 on the support cradle 57 makes it possible to tip the heater block for improved viewing and to agitate the heater block during the course of a heating operation. It is important that the trunnions 51 and 52 be formed from insulating material. If metal trunnions are employed, the heat loss from the ends of the block may well be sutficient to reduce the temperature of the two end cavities 12A and 12B below that of the interior cavities 12. This, of course, may result in inaccurate test results with respect to any test tubes mounted at the ends of the heater block 11 and is therefore undesirable.

FIGS. 4 and 5 illustrate a test tube heater 110 which comprises a second embodiment of the present invention. The heater 110 is constructed primarily for use on high temperature applications, whereas the heater 10 described hereinabove is intended primarily for relatively low temperature applications. More particularly, the heater 10 of the first described embodiment is intended primarily for use in tests and other heating applications in which the test tubes and their contents are maintained at a constant temperature below C., whereas the heater is adapted for use in tests running to substantially higher temperatures.

In many respects, the heater 110 is essentially similar to the heater 10. Thus, the heater 110 comprises a heater block 111 having a plurality of elongated test tube receiving cavities 112, the construction of the block being best shown in FIG. 4. A test tube 14 is shown mounted in one of the cavities 112 at the left-hand end of the block. As before, the heater block 111 is mounted upon a hollow sheet metal base 16, the bottom of each of the cavities 112 being closed by the top plate 13 of the base. A pair of insulated trunnion members 51 and 52 are mounted on the heater block 111 by suitable means such as the screws 53 and 54. As before, the trunnions are utilized to support the main heater on the upstanding legs 55 and 56 of a support cradle 57. The support cradle is provided with a plurality of insulated feet 58. A similar plurality of insulated feet 48 are mounted on the bottom plate as of the heater base 16 to permit use of the heater independently of the support cradle 57.

The test tube heater 119 is provided with a resistancewire heater 122 that is mounted immediately below the top plate 13 of the base 16 (see FIGS 4 and 5). The

heater 122 is essentially similar to the heater 22, except that it is constructed to afford a substantially greater heat output than the previously described embodiment. Again, the heater may be supported in heat-transferring relation to the plate 13 by means of vertically extending flanges which form a part of the bottom cover 46 of the base 16.

The heater block 111 is provided with a plurality of viewing apertures 117 which extend through the block. These apertures or windows 117 correspond in number to the cavities 112 and extend through the cavities to enable the user of the heater to observe the contents of test tubes or similar vessels being heated therein. Windows 117 are generally similar to the windows 17 in the heater 10 (FIGS. 1 and 2) but are substantially smaller, being confined to the very bottom portion of each of the cavities 112. If necessary for effective inspection of the test tube cavities, the windows could be located higher up in the block 111. The size of the windows 117 effectively limits air circulation therethrough and makes it possible to omit taransparent covers for the windows in this embodiment of the invention. On the other hand, the transparent cover construction shown in FIGS. 1 and 2 is preferred, particularly if it is necessary to enlarge the windows 117 to any substantial size.

The control system for the heater 110, which is best illustrated in FIG. 5, comprises a pair of actuating members 124 and 125 which are essentially V-shaped in configuration and which have their apex portions 126 and 127 aligned with each other. The actuating members 124 and 125 are rigidly affixed, as by spot welding, to a pair of mounting blocks 128 and 129 located adjacent the opposite ends of the heater block 111 As before, the mounting blocks are directly and rigidly secured to the heater block by suitable means such as the mounting screws 132 and 133 so that the mounting blocks move toward and away from each other directly in response to expansion and contraction of the heater block.

A mounting stud 135 is mounted upon the actuating member 124, being insulated from the member 124 by an insulating bushing 136. A contact arm 137 is mounted upon the mounting stud 135 but is electrically insulated with respect thereto by means of an insulating bushing 138. A first electrical contact element 139 is mounted upon the cantilever arm 137, an electrical connector 141 being provided for the contact element 139. The contact arm 137 extends beyond the contact element 139 and is engaged by a first adjustment member comprising an adjusting screw 142 that is mounted upon a bracket 143, the bracket 143 being aflixed to the second actuating member 125 of the control device.

The control arrangement illustrated in FIG. further includes a second contact element 144 that is mounted upon a flexible cantilever spring 145. The flexible spring 145, in turn, is aflixed to a relatively stiff cantilever spring arm 146 that is mounted upon and extends outwardly of the mounting stud 135, being electrically connected thereto. A snap action adjusting member, comprising an adjusting screw 147, is mounted upon the relatively stilt spring member 146 and engages the free end of the flexible spring arm 145 to aiiord a means for adjusting the free end of the spring 145, and hence the contact 144, toward and away from the first contact 139. A second adjusting member, comprising a threaded shaft 148, is mounted in a bushing 149 on the actuating member 124 and extends through the bushing into engagement with the relatively stiff spring arm 146. The tip 151 of the adjusting member 148 is formed of insulating material so that the adjusting member is electrically insulated from the operating circuit of the control device.

The control system illustrated in FIG. 5 further includes a magnetic snap action control means. This snap action control meatns comprises a permanent magnet 152 of generally annular configuration disposed in encompassing relation to the first electrical contact element 139.

'5 The snap action control further includes a keeper member 153 of substantially disc-like configuration which is disposed in encompassing relation to the contact 144. The permanent magnet 152 is magnetized in a direction to attract the keeper member 153 and maintain the contacts 139 and 144 normally in contact with each other.

The operation of the embodiment of FIGS. '4 and 5 is essentially similar to that of FIGS. 1-3 except that the control arrangement illustrated in FIG. 5 is a rapid makeand-break device as contrasted with the slow make-and break control system shown in FIG. 3. The use of a snap action control in a high temperature heater is highly desirable, because the current drawn by the heater is relatively large and a slow make-and-break device may produce excessive arcing and consequent wear and deteriora tion of the contacts. 7

When the heater 111i is placed in operation, the heating element 122 of the heater is energized through an operating circuit whiohfincludes, in series, the contacts 139and 144. As noted hereinabove, these contacts are normally disposed in engagement with each other. With continued operation of the heating element 122, the block 1111 increases in temperature and expands. As a consequence, the mounting blocks 128 and 129 move away from each other, stretching the actuating members 124 and and tending to flatten these members. As a result, the contact 139 is pulled downwardly, as seen in FIG. 5, since the arm 137 is constrained to move conjointly with the actuating member 125 due to engagement of the arm 137 by the adjusting screw 142 mounted on the bracket 143. The other contact arm, comprising the spring members and 146, [tends to move in the opposite direction in accordance with the movement of the actuating member 124. Thus, the two arms tend to pull the contacts 139 and 144 apart and, when the force applied to the arms is suflicient to overcome the magnetic attraction between the magnet 152 and the magnetic keeper 153, the contacts snap open.

As soon as the contacts 133 and 144 are disengaged, the operating circuit for the heater 122 is broken. Subsequently, the heater block 111 cools, since it is no longer being heated by the heating element 122. As the block cools, the two mounting blocks 128 and 129 move back toward each other, due to contraction of the heater block, again bending the two V-shaped actuating members toward each other. When the actuating members 124 and 125 have moved close enough to each other for the magnetic attraction between the members 152 and 153 to overcome the spring bias of the flexible spring arm 145, the contacts again snap closed and the heating cycle is re-initiated.

The three adjusting members 142, 147 and 148 of the control system illustrated in FIG. 5 make it possible to obtain highly accurate and effective control of operation of the heater 110. The initial position of .the contact 139 is adjusted by means of the adjusting screw 142. The position of the contact 144 relative to the stiff cantilever spring arm 146 is adjusted by means of the adjusting member 147. This adjustment is utilized to obtain etfective snap action in operation of the control. Finally, the adjusting member 148 can be utilized to control the effective operating range of the contacts 139 and 144, thereby controlling the duty cycle of the heater. This control arrangement makes it possible to maintain the heater block 111 at a relatively high constant temperature, within close limits, for indefinite periods of time. Furthermore, and as in the embodiment of FIG. 1, the temperature of the block 111 is held constant regardless of changes in ambient temperature, since the eiiect of ambient temperature on the heater block'111 and on the control system is reflected in operation of the actuating members 124 and 125 in the same manner as temperature changes induced by the heating element 122. The snap action mechanism of the control device illustrated in FIG. 5 is substantially similar, in many respects, to

the control apparatus described and claimed in the copending application of Cleophas B. Anderson, Darrlle D. Moore and William J. Walsh, Serial No. 856,164, filed November 30, 1959, now Patent No. 2,960,588, issued November 15, 1960, and reference may be had to that patent for a more complete description of the operational advantages and structural features of the snap action portion of the control.

FIG. 6 illustrates another test tube heater 210, and specifically the control apparatus of the heater, the heater 214 being substantially similar in most respects to the embodiment of FIGS. 1-3. Thus, the control arrangement shown in FIG. 6 includes the two mounting blocks 28 and 29, mounted upon a suitable workpiece such as the heater block 1 1 of FIG. 1' for displacement relative to each other in response to expansion and contraction of the workpiece. As before, the two actuating members 24 and 25 are mounted upon the mounting blocks 28 and 2 9 with the apex portions 26 and 27 disposed immediately opposite each other.

In the construction shown in FIG; 6, a relatively small enclosed switch 211 having a control member 2 12 is mounted upon the actuating member 25 and is connected in series in the energizing circuit for the heating element 2 2. An adjustable switch actuating element 213 is mounted upon the actuating member 24 in electrically insulated relation thereto and extends into engagement with the switching control member 212 of the switch 211. The actuating element 213 can be adjusted toward and away from the switching member 212.

When the heater 210 is placed in operation, the heating element 22 is energized through the operating circuit which includes, in series, the switch 211. The switch 211 is normally held in closed condition by engagement of the switch actuating element 213 with the switching member 212. As the workpiece heated by the heating element 22 expands, the mounting blocks 28 and 29 that are secured to the workpiece move away from each other, eifeotively flattening the two V-shaped actuating members 24 and 25. As a consequence, the switch 211 and the switch actuating element 2113 move away from each other until, eventually, the actuating element releases the switching member 212, permitting the switch 211 to open. As a consequence, the heating element 22 is deenergized and the workpiece being heated begins to cool and contract. Accordingly, the actuating members 24 and 25 are slowly returned toward their original configuration, with the result that the actuating element 213 again engages the switching member 212 and closes the switch 211 to initiate another heating cycle. It is thus seen that the construction of FIG. 6 operates in essentially the same manner as the control arrangement described hereinabove in connection with FIG. 3.

Hence, while preferred embodiments of the invention have been described and illustrated, it is to be understood that they :are capable of variation and modification.

I claim: 7

l. A test tube heater comprising: an aluminum heater block having a dark anodized surface and having a plurality of elongated cavities therein for receiving individual test tubes of given length, said cavities having a height greater than one-half said length, said heater block further having a corresponding plurality of viewing apartures each extending through only the base portion of a respective one of said cavities to permit visual inspection and cleaning of the base portion of said cavity; a pair of cover members ct transparent heat resistant material removably mounted on opposite sides of said block to prevent air circulation through said viewing apertures and said cavities; and heating means, at the base of said block, for heating said block and the contents thereof to a predetermined temperature.

2. A test tube heater comprising: a metal heater block having a plurality of elongated cavities therein for receiving individual test tubes, said heater block further having a corresponding plurality of viewing apertures extending transversely through said cavities and permitting visual inspection of the base portion of each of said test tube receiving cavities; a heater base, mounted on said block, and closing ofi the base ends of said cavities, heating means, mounted on said base, for heating said block and the contents thereof to a predetermined temperature; a support cradle; means for pivotally mounting the heater block, at its ends, upon said cradle; and insulating means for inhibiting heat transfer lfIOl'l'l said heater block to said cradle to prevent excessive temperature dilierentials between diiierent cavities in said block.

3. A test tube heater comprising: a high conductivity metal heater block having a plurality of elongated cavities therein for receiving individual test tubes of given length, said cavities having a height greater than one-half said length, said heater block further having acorresponding plurality of viewing apertures extending through the base portion of said block, through said cavities, to perrnit visual inspection and cleaning of the base portion of each of said test tube receiving cavities; a pair of heat [resistant transparent covers removably mounted on opposite sides of said block to prevent air circulation through said viewing apertures and said cavities; electrical heating means, at the base of said block, for heating said block and the contents thereof to a predetermined temperature; a pair of V-shaped actuating members, each having a thermal coefiicient of expansion substantially smaller than that of said block, each afiixed at its opposite ends to said block with the apices of the members aligned with each other, said members each being subject to substantial deformation from mechanical stresses applied thereto in response to expansion and contraction of said workpiece, said members each being partially cut away at the apex to afford a hinge portion; and a control switch, interposed between said actuating members and actuatable between an open condition and a closed condition in response to predetermined deformations of said members, said control switch being electrically con nected to said electrical heating means to control energization of the heating means and maintain the block at a constant temperature.

4. A thermal control for controlling the temperature of a workpiece having a given thermal coefiicient of expansion, said control comprising: a pair of actuating members having a thermal coefiicent of expansion substantially different from that of said workpiece, afiixed directly to said workpiece, said actuating members each having a configuration subject to substantial deformation trom mechanical stresses applied thereto in response to expansion and contraction of said workpiece; a first electrical contact element, mounted on one of said actuating members and movable i roma given normal position in response to deformation of said members; a second electrical contact element; resilient support means mounting said second contact element in position to engage said first contact element when in said normal position and effective to disengage said second contact element from said first contact element when said first contact element is moved through a predetermined distance; magnetic control means for eifecting a snap action in engagement and disengagement of said contacts, said magnetic control means comprising a cup-shaped permanent magnet member mounted in encompassing relation to one of said first and second contacts and a ferro-imagnetic keeper member mounted adjacent the other of said contact elements, said magnet and keeper members forming a magnetic structure substantially totally encompassing the contact surfaces of said contact elements and affording a magnetic field tending to dampen arcing between said contacts; and independent gross and fine adjustment devices, engaging said resilient support means, for adjusting the effective operating range of said contacts to adjust the duty cycle of the device and for adjusting the initial starting position 11 of said second contact, respectively, said fine adjustment device being mounted on the other of said actuating members.

5. A thermal control for controlling the temperature of a test tube heater block or like workpiece having a given thermal coeificient of expansion, said control comprising: a pair of V-shaped actuating members, having a thermal coeflicient of expansion substantially difierent from that of said workpiece, affixed directly at their ends to said workpiece with their apices aligned with each other, and subject to substantial deformation from mechanical stresses applied thereto in response to expansion and contraction of said workpiece; a first electrical contact element, mounted on one of said actuating members and movable from a given normal position in response to deformation of said members; a second electrical contact element; an adjustable positioning member, mounted on the other of said actuating members and effectively engaging said first contact element, for determining the normal position of said fir-stcontact element; resilient support means mounting said second contact element in position to engage said first contact clement when in said normal position and efiective to disengage said second contact element from said first contact element when said first contact element is moved through a predetermined distance; magnetic control means for efiecting a snap action in engagement and disengagement of said contacts,

said magnetic control means comprising first and second magnetic members mounted adjacent said first and second elements, respectively, and forming a magnetic field substantially totally encompassing the contact surfaces of said contact elements; means for adjusting the efliectivel operating range of said contacts to adjust the temperature of the device; and means for independently adjusting the initial starting position of said second contact.

References Cited in the file of this patent UNITED STATES PATENTS 1,535,751 Turenne Apr. 28, 1925 2,184,270 Campbell Dec. 26, 1939 2,248,666 Fischer July 8, 1941 2,313,015 Hesse Mar. 2, 1943 2,478,344 Valverde Aug. 9, 1949 2,562,821 Rothweiler July 31, 1951 2,736,779 Mathiseu et al. Febf28, 1956 2,753,420 Rosenberg et al. July 3, 1956 2,932,718 Marsters Apr. 12, 1960 2,959,665 Rhees Nov. 8, 1960 FOREIGN PATENTS 14,300 Great Britain Feb. 7, 1907 388,393 Germany Jan. 12, 1924 987,101 France Jan. 24, 1951 151,920 Australia June 17, 1953 

1. A TEST TUBE HEATER COMPRISING: AN ALUMINUM HEATER BLOCK HAVING A DARK ANODIZED SURFACE AND HAVING A PLURALITY OF ELONGATED CAVITIES THEREIN FOR RECEIVING INDIVIDUAL TEST TUBES OF GIVEN LENGTH, SAID CAVITIES HAVING A HEIGHT GREATER THAN ONE-HALF SAID LENGTH, SAID HEATER BLOCK FURTHER HAVING A CORRESPONDING PLURALITY OF VIEWING APARTURES EACH EXTENDING THROUGH ONLY THE BASE PORTION OF A RESPECTIVE ONE OF SAID CAVITIES TO PREMIT VISUAL INSPECTION AND CLEANING OF THE BASE PORTION OF SAID CAVITY; A PAIR OF COVER MEMBERS OF TRANSPARENT HEAT RESISTANT MATERIAL REMOVABLY MOUNTED ON OPPOSITE SIDES OF SAID BLOCK TO PREVENT AIR CIRCULATION THROUGH SAID VIEWING APERTURES AND SAID CAVITIES; AND HEATING MEANS, AT THE BASE OF SAID BLOCK, FOR HEATING SAID BLOCK AND THE CONTENTS THEREOF TO A PREDETERMINED TEMPERATURE. 